The present invention relates to polycrystalline masses of self-bonded diamond particles (i.e. polycrystalline compacts) useful as tool components and more particularly to a metal coated polycrystalline mass with enhanced oxidation resistance.
It is well known to use diamond, cubic boron nitride (CBN) or other abrasive particles embedded in the grinding, abrading, or cutting section of various tools. The active sections of such tools include resin bond and metal bond construction. Such abrasive particles have been coated with various metals and alloys of metals in single or multiple layers in order to enhance bond retention, improve high temperature oxidation resistance, suppress high temperature graphitization, and like benefits. Such coatings are especially useful when fine-grain diamond or other abrasive grits are employed in the various tools. Representative art in this single grain coating endeavor include British Pat. Nos. 1344237 and 712057, U.S. Pat. Nos. 2,367,404, 3,650,714, 3,957,461, 3,929,432, 3,984,214, and German Offenlegungsschrift No. 2124637.
Also well known in this art are compacts of polycrystalline abrasive particles typified by polycrystalline diamond and polycrystalline CBN compacts. Such compacts are represented by U.S. Pat. Nos. 3,745,623 and 3,609,818 with respect to polycrystalline diamond compacts and U.S. Pat. Nos. 3,767,371 and 3,743,489 with respect to polycrystalline CBN compacts. While such polycrystalline compacts represent a significant contribution to the art in many fields of use, thermal degradation at elevated temperature, e.g. above about 700.degree. C., did limit their usefulness, especially in metal matrix bond applications. The thermal stability of such polycrystalline compacts was improved with the advent of porous self-bonded diamond and CBN compacts containing less than about 3% non-diamond phase, hereinafter termed "porous compact". Compacts of this type are the subject of U.S. Pat. Nos. 4,224,380 and 4,288,248.
Since, on a microscale, the surface of porous compacts is extremely rough, bond retention by mechanical means generally is adequate; hence, the art has not recognized a general need for a matrix bond reactive coating as is the case with a microcrystalline counterpart. Additionally, the excellent thermal stability property possessed by the noted self-bonded diamond particles with an interconnected network of pores dispersed throughout is postulated to be due to the removal of metallic sintering aid normally found in such compacts which metallic substance possesses a different coefficient of thermal expansion than is possessed by the diamond. Thus, it was theorized that application of a matrix bond reactive coating could subject the porous compact to possible reinfiltration by the coating metal with consequent loss of thermal stability occasioned thereby.
An additional factor militating against application of a matrix bond reactive coating is the expected stability of such compact to not oxidize at higher temperatures of processing required in metal bond formation. Oxidation stability is not a recognized problem of conventional compacts. Moreover, larger single-crystal diamond of comparable dimension is known to possess fairly good oxidation stability due to their large size since diamond oxidation is a function of temperature, time, and state of subdivision (surface area per unit weight).